1. Field of the Invention
The present invention relates to a spin valve sensor with a free layer structure having a cobalt niobium or cobalt niobium hafnium layer which has a negative magnetostriction for counterbalancing a positive magnetostriction of the remaining of the layers in the free layer structure.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer structure and a ferromagnetic free layer structure. An antiferromagnetic pinning layer interfaces the pinned layer structure for pinning a magnetic moment of the pinned layer structure 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the magnetic disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer structure is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or bias point position in response to positive and negative magnetic field signals from a rotating magnetic disk. The quiescent position, which is preferably parallel to the ABS, is the position of the magnetic moment of the free layer structure with the sense current conducted through the sensor in the absence of signal fields.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layer structures are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons are scattered at the interfaces of the spacer layer with the pinned and free layer structures. When the magnetic moments of the pinned and free layer structures are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. Changes in scattering changes the resistance of the spin valve sensor as a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layer structures. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in the resistance of the sensor as the magnetic moment of the free layer structure rotates from a position parallel with respect to the magnetic moment of the pinned layer structure to an antiparallel position with respect thereto and R is the resistance of the sensor when the magnetic moments are parallel.
In addition to the spin valve sensor the read head includes nonconductive nonmagnetic first and second read gap layers and ferromagnetic first and second shield layers. The spin valve sensor is located between the first and second read gap layers and the first and second read gap layers are located between the first and second shield layers. In the construction of the read head the first shield layer is formed first followed by formation of the first read gap layer, the spin valve sensor, the second read gap layer and the second shield layer. Spin valve sensors are classified as a top or a bottom spin valve sensor depending upon whether the pinning layer is located near the bottom of the sensor close to the first read gap layer or near the top of the sensor close to the second read gap layer. Spin valve sensors are further classified as simple pinned or antiparallel pinned depending upon whether the pinned layer structure is one or more ferromagnetic layers with a unidirectional magnetic moment or a pair of ferromagnetic layers that are separated by a coupling layer with magnetic moments of the ferromagnetic layers being antiparallel. Spin valve sensors are still further classified as single or dual wherein a single spin valve sensor employs only one pinned layer and a dual spin valve sensor employs two pinned layers with the free layer structure located therebetween.
The free layer structure typically employs a nickel iron layer which provides a desirable magnetic softness for the free layer. This means that the free layer has a low uniaxial anisotropy HK which promotes responsiveness of the free layer structure to signal fields from a rotating magnetic disk. When the free layer structure is highly responsive a small signal field will rotate the magnetic moment of the free layer structure which causes a change in the magnetoresistance of the spin valve sensor. It has been found that when the free layer structure also includes a cobalt iron or cobalt layer, sometimes referred to as a nanolayer, between the nickel iron layer and the spacer layer and interfacing the spacer layer that the magnetoresistance or magnetoresistive coefficient dr/R is improved. In order to obtain a desirable increase in the magnetoresistive coefficient dr/R, it has been further found that the thickness of the cobalt iron or cobalt layer should be at least 10xc3x85. Unfortunately, any thickness of the cobalt iron layer reduces the softness of the free layer structure so that it is not as responsive to signal fields from the rotating magnetic disk. A cobalt based film, such as cobalt (Co) or cobalt iron (CoFe), has a magnetic moment of approximately 1.7 times the magnetic moment of nickel iron (NiFe) for a given thickness. Accordingly, an increase in the ratio of the thickness of the cobalt iron or cobalt layer to the thickness of the nickel iron layer increases the uniaxial anisotropy HK of the free layer structure and reduces its softness so that it is less responsive to signal fields. Uniaxial anisotropy field is the amount field required to rotate the magnetic moment of the free layer from a position parallel to the ABS to a position perpendicular thereto. One way to overcome the increase in uniaxial anisotropy HK of the free layer structure, because of an increase in the thickness of the cobalt iron or cobalt nanolayer, is to increase the thickness of the nickel iron layer so as to reduce the above-mentioned ratio. Unfortunately, this reduces the linear bit density of the read head which is the number of magnetic bits which can be read linearly along a track of a rotating magnetic disk.
Further, any increase in the ratio of the thickness of the cobalt or cobalt iron layer to the thickness of the nickel iron layer causes the free layer structure to have a hysteresis. This hysteresis is indicated in a hysteresis loop which is a graph of the magnetic moment M of the free layer in response to an applied field H (signal field) directed perpendicular to the ABS. The hysteresis loop, which is referred to as the hard axis loop, has an opening due to the hysteresis which can be on the order of 5 to 7 oersteds. The opening in the hard axis loop is quantified as hard axis coercivity HC which is measured from the origin of the x and y axes to the intersection of the loop with the x axis (applied signal). It has been found that when the hard axis coercivity is high the head generates Barkhausen noise which is due to the fact that the magnetic domains of the cobalt based layer are oriented in different directions. Accordingly, as the signal fields rotate the magnetic moment of the free layer some of the magnetic domains do not follow the directions of the signal fields. The magnetic domains that do not readily follow the signal field direction follow behind the signal field direction in an erratic behavior, referred to as jumps in their movements, which causes the aforementioned Barkhausen noise. This Barkhausen noise is superimposed upon the playback signal which is unacceptable.
The aforementioned hysteresis is caused by a positive magnetostriction (+MS) of the cobalt or cobalt iron layer. After fabrication of all of the layers of rows and columns of read heads on a wafer, the wafer is diced into rows and each row is lapped (a grinding process) to form an air bearing surface for each magnetic head. After lapping the row of magnetic heads, the magnetic heads are diced into individual magnetic heads. Unfortunately, the lapping process causes the magnetic head to be in compression at the ABS. Because of the positive magnetostriction of the cobalt or cobalt iron layer the magnetic moment thereof is urged from a parallel position with respect to the ABS toward a perpendicular position with respect thereto. Accordingly, the positive magnetostriction of the cobalt or cobalt iron layer causes the aforementioned hysteresis and is not a desirable ingredient for the free layer structure. With this ingredient the free layer structure is in a multi-domain state which causes a magnetic moment of the free layer structure to be unstable and to move in a jumping fashion. The result is the aforementioned Barkhausen noise and irreproducible signals.
If the hysteresis or opening in the hard axis loop could be eliminated the aforementioned moment versus applied field graph (M/H graph) of the responsiveness of the spin valve sensor would be simply a straight line. This straight line, which is the transfer curve of the read head, indicates that the read head will be magnetically stable upon the application of the signal fields. It is an object of my invention to eliminate or reduce the positive magnetostriction of the free layer structure so that the hysteresis of the moment of the free layer structure is eliminated or minimized.
In the present invention the free layer structure includes a first free layer composed of cobalt or cobalt iron that interfaces the spacer layer and a second free layer composed of cobalt niobium or cobalt niobium hafnium. The cobalt niobium or cobalt niobium hafnium layer has a negative magnetostriction which can be made to completely counterbalance or least partially counterbalance the positive magnetostriction of the cobalt or cobalt iron layer. Accordingly, with the present invention the cobalt or cobalt iron layer may still be employed next to the spacer layer for increasing the magnetoresistive coefficient dr/R of the spin valve sensor. In another aspect of the invention the free layer structure includes a third free layer of nickel iron with the nickel iron layer being located between the cobalt or cobalt iron layer and the cobalt niobium or cobalt niobium hafnium layer. With this arrangement the nickel iron layer will provide magnetic softness for the free layer structure and will not be affected by the texture of the cobalt niobium or cobalt niobium hafnium layer on top thereof and fabricated subsequent thereto. The cobalt in the cobalt niobium or cobalt niobium hafnium layer should be at least 90%. The niobium causes the layer to have a negative magnetostriction and the hafnium causes the layer to have a positive magnetostriction. Accordingly, the negative magnetostriction may be achieved with niobium only with the layer being cobalt niobium such as Co95Nb5. When this ratio is used the layer can be very thin so as to properly balance the negative magnetostriction against the positive magnetostriction of the remainder of the layers in the free layer structure. Alternatively, the layer may be thicker when hafnium is employed with an exemplary layer being Co92Nb5Hf3.
Another aspect of the present invention is that the free layer structure consist of only a cobalt or cobalt iron free layer and a cobalt niobium or cobalt niobium hafnium layer. Such a free layer structure would not have a nickel iron free layer which would permit a significant reduction in the thickness of the free layer structure for promoting the linear read bit density of the head. The cobalt niobium or cobalt niobium hafnium layer can be provided with a proper ratio and thickness so that its positive magnetostriction overcomes the negative magnetostriction of the cobalt or cobalt iron layer with a slight amount of negative magnetostriction remaining. The small remaining negative magnetostriction will provide the free layer structure with a small uniaxial anisotropy oriented parallel to the ABS which can be easily rotated by signal fields from the rotating magnetic disk. Accordingly, the free layer structure has a desirable magnetic softness or sensitivity to the signals from the rotating magnetic disk. After construction of the rows and columns of magnetic heads on the wafer, the wafer may be subjected to a longitudinal field (parallel to the ABS) in the presence of heat for rotating the magnetic moment of the cobalt niobium or cobalt niobium hafnium layer parallel to the ABS, which rotation also aligns the magnetic moment of the cobalt iron or cobalt layer parallel to the ABS. In this aspect of the invention, the cobalt or cobalt iron layer interfaces the spacer layer and is located between the spacer layer and the cobalt niobium or cobalt niobium hafnium layer.
An object of the present invention is to reduce the positive magnetostriction of a free layer structure when the free layer structure employs a cobalt or cobalt iron layer next to a spacer layer.
A further object is to provide a magnetically soft free layer structure which does not employ a nickel iron free layer.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.